A typical automotive air conditioning system includes a compressor, a condenser, an expansion device, and an evaporator. Hydraulically connecting the aforementioned components in series are refrigerant tubes that are capable of conveying high and low pressure refrigerant flows. A two phase refrigerant used in a modern automotive air conditioning system is an environmentally friendly refrigerant known as R-134a and low Global Warming Potential (GWP) refrigerants such as HFO-1234yf.
The compressor is commonly referred to as the heart of the air conditioning system in which it is responsible for compressing and transferring the refrigerant throughout the system. The compressor includes a suction side and a discharge side. The suction side is referred to as the low pressure side and the discharge side is referred to as the high pressure side.
The evaporator is disposed in the passenger cabin of the automobile and the condenser is disposed in the front portion of the engine compartment or more precisely, in front of the radiator. Within the evaporator, cold low pressure liquid refrigerant boils by absorbing heat from the passenger compartment. The low pressure vapor refrigerant exiting from the evaporator is drawn and compressed by the compressor into a high temperature vapor refrigerant. The compressed high temperature vapor refrigerant is then discharged by the compressor to the condenser. As the high pressure vapor refrigerant passes through the condenser, the refrigerant is condensed to a high pressure lower temperature liquid refrigerant as it releases the heat it absorbed from the passenger cabin to the ambient air outside of the passenger cabin. Exiting the condenser, the high pressure liquid refrigerant passes through an expansion device that regulates the flow of the high pressure liquid refrigerant to the evaporator to repeat the process of heat transfer from the cabin to the outside ambient air.
The temperature of the returning low pressure vapor refrigerant to the compressor from the evaporator is typically 40° F. to 100° F. lower than the high pressure liquid refrigerant exiting the condenser. An internal heat exchanger, such as a double pipe counter-flow heat exchanger, is known to be used to take advantage of the temperature differential between the low pressure low temperature vapor refrigerant and the high pressure high temperature liquid refrigerant to improve the overall cooling capacity of the air conditioning system. The double pipe heat exchanger includes an outer pipe and an inner pipe co-axially located within the outer pipe. The diameter of the inner pipe is smaller than the diameter of the outer pipe, thereby defining an annular gap between the inner pipe and outer pipe for refrigerant flow. The relatively cooler low pressure vapor refrigerant exiting the evaporator is passed through the annular gap and the relatively hotter liquid refrigerant exiting the condenser is passed through the inner pipe. Heat is transferred from the high pressure liquid refrigerant exiting the condenser to the cooler low pressure vapor refrigerant returning to the compressor in the internal heat exchanger. By decreasing the temperature of the high pressure liquid refrigerant prior to its flowing through the expansion device, the expansion device may be set at a lower temperature; therefore the temperature of the refrigerant entering the evaporator is at a lower temperature. A SAE International Publication No. 2007-01-1523 has shown that an internal heat exchanger such as the one described above can increase the amount of internal heat exchange from 390 W to 550 W; thereby improving the cooling performance of the air conditioning system.
The internal heat exchanger describe above has its disadvantages. The installation of such a heat exchanger into an engine compartment is difficult due to the limited amount of space within an engine compartment. Furthermore, such a double pipe heat exchanger is also known for low heat transfer efficiency and high pressure drop. It is therefore desirable to have an internal heat exchanger that is compact, but with a high heat transfer effectiveness and low pressure drop. It is further desirable to have a compact internal heat exchanger that is robust during normal operating conditions. It is still further desirable to have a compact internal heat exchanger that is cost effective to manufacture.